Make _active_tasks more flexible

This proposal is simply to allow the output of _active_tasks to be less rigid. Basically to allow each task to be able to output different JSON fields.

Somethings like the status text simply go away. Instead application can built it based on more granular fields provided by _active_tasks.
Some examples:

1) "progress" (an integer percentage, for all tasks)

2) "database" (for compactions and indexer tasks)

3) "design_document" (for indexer and view compaction tasks)

4) "source" and "target" (for replications)

5) "docs_read", "docs_written", "doc_write_failures",
"missing_revs_found", "missing_revs_checked", "source_seq",
"checkpointed_source_seq" and "continuous" for replications